In the brain, the topography will be determined of neurons which specifically bind and are sensitive to hormones regarding the regulation of endocrine gland functions and hormone related behavior. Radioactively labeled hormones such as estradiol, testosterone, progesterone, cortisol, or congeners of them, will be injected and dry- mount autoradiograms of the brain prepared. Maps of the cellular hormone distribution will be provided, based on the autoradiographic data obtained. The chemical nature of the radioactivity will be identified. Different vertebrate species, representative of different phylogenetic genera, as well as different ontogenetic states of at least one mammalian species, will be studied and the results compared. The information of "hormone-neuron" distribution will be utilized in stereotaxic lesion and implantation experiments and the significance of the hormone binding sites assessed by estimation of related changes in the blood level of trophic hormones using radioimmunoassay and behavioral tests. Basic and new information on brain organization and function is expected. Both qualitative and quantitative autoradiographic studies are aimed at finding possible sex and age differences in neuronal hormone binding and hormone action on the brain. Efforts will be made to develop a technique for electron microscope autoradiography to be applicable to the study of subcellular hormone distribution.